Porous metallic membranes are of great interest and high demand in industry. Their applications range from hydrogen storage, gas filtration such as hydrogen purification, fuel cell technology, membrane reactor processes and environmental water filtration.
Metallic membranes address a number of drawbacks associated with porous polymer membranes. For instance, due to the methods used in their production, polymer membranes typically contain truncated, irregular pores with large variations in pore sizes and face limitations in the control of pore dimension and geometry. Specifically, a polymer membrane may be composed of multiple layers, wherein the pores of each layer are not aligned with the pores of an adjacent layer, resulting in an irregular path for filtrate movement. Furthermore, at least one or more layers may possess pores in the nano-scale dimension. These factors can cause the polymer membrane to exhibit high flow resistance. As a result, polymer membranes typically offer lower selectivity and flux relative to metallic membranes. Furthermore, membranes made from polymer materials are not able to operate under harsh environments (such as high pressure conditions) without impairment to its separation quality or suffering damage to its structure.
In comparison, the use of metallic materials to manufacture porous metal membranes is advantageous. For example, when applied to the synthesis of a fuel cell, metallic membranes provide an attractive combination of electrical conductivity, higher flux, efficiency, selectivity and mechanical durability when compared to a polymer membrane.
At present, palladium or palladium alloys are commonly used in the synthesis of porous metal membranes, e.g., for hydrogen gas filtration and storage. In particular, palladium and its alloys are selected for their good permeability to hydrogen and further because palladium does not experience embrittlement at high hydrogen partial pressures. However, the scarcity of palladium makes it an expensive raw material and renders it prohibitively expensive to use in large scale manufacture of metallic membranes.
Apart from cost, the current processes available in the art for forming metallic membranes are also non-optimal. For instance, one known process for fabricating metallic membranes is electroless plating. However, a drawback of this process is that there is wastage of the material that is to be plated. For instance, metal that precipitates out of solution may not be deposited on the substrate metal. Instead, precipitated metal may collect in the electroless bath in the form of particulate dust or may even deposit onto surfaces other than the desired substrate metal, such as the surface of the container holding the plating bath.
Another process that has been used to prepare metal membranes is sputtering. However, sputtering requires stringent control of the process conditions (e.g. vacuum), and is more suited for depositing thin layers over small surface areas. Furthermore, it is a relatively slow process compared to electroless plating, making it impractical for use in large-scale manufacturing. The high temperature conditions used in the sputtering process further renders it unsuitable for depositing metals on substrates having low melting points.
Traditional electroplating processes combined with UV lithography techniques create smooth, highly uniform membranes with high resolution. However, a major drawback is the high cost of such processes. Known UV lithography techniques can produce pore sizes of several microns or less, while deep UV lithography techniques can produce even smaller pores, but these techniques are also very costly. In addition, membranes generated by UV lithography techniques are limited to having cylindrically shaped pores.
In environmental filtration applications, large volumes of sample have to be filtrated with high precision. Membranes with even one pore slightly bigger than the rest can allow contaminants to pass through. Thus, there is a need to synthesize porous membranes with high flux, and high degrees of precision. Thus, there is a need to develop a process that allows for control of the geometry and size of the pores of porous membranes.
As such, there is a need to provide a process that provides porous metal membranes that overcome, or at least ameliorate, one or more of the disadvantages described above.